Thermal inkjet printing may be briefly described as an ink-drop on demand type of printing which uses thermal energy to produce a vapor bubble in an ink-filled channel. Each printhead supports a nozzle plate that is perforated with a large number of nozzle orifices distributed over a precise geometric pattern. Each orifice is served by a respective ink supply channel. Each of these respective ink supply channels is served by a rapidly responding electrical heating element such as a resistor located in the channel near the respective nozzle. These heating elements are individually charged by specifically addressed electrical pulses to momentarily vaporize a small quantity of ink in the channel. The abrupt phase change of the ink from liquid to vapor results in an abrupt volume increase in the proximity of the heater. This resulting vapor volume is characterized as a bubble. This sudden volumetric expansion of a bubble within the closed confines of the supply channel is accommodated toward the nozzle orifice by displacement of a droplet quantity of liquid ink from the nozzle orifice as a bulge of liquid that is held by surface tension to the liquid column behind it and to the nozzle face.
As the vapor cools and condenses, the bubble collapses. The segment of ink within the nozzle channel between the collapsing bubble and the liquid bulge reverses flow direction to fill the volumetric void. This flow direction reversal of ink mass causes a concentration of tensile stress between the ink mass at the inner end of the nozzle segment near the collapsing bubble and the accelerating mass of the ink bulge from the nozzle orifice thereby separating the bulge from the segment inner end as a droplet. This acceleration of the ink bulge mass out of the nozzle while the bubble volume is growing provides the velocity and momentum to carry the droplet in a substantially straight line toward an intended target medium such as a sheet of paper drawn over a platen.
Size of an ink droplet propelled from a nozzle orifice is largely determined by the temperature and viscosity characteristics of the ink. For print contrast consistency, some control over the ink temperature is asserted by means of one or more additional heating elements that are positioned in heat transfer association with the body of the printhead within which the ink flow channels are formed. This body is a laminated composite of numerous substrates, each having a distinctive topology of nozzles, fluid flow channels, chemically deposited conductors, and solid state circuitry constituents. Integrated with this topology is a series of electrical resistors having an operational purpose only during the printhead manufacture for monitoring and testing the quality of production.
To achieve manufacturing efficiency, electrical connection terminals are minimized by imposing multiple utilities upon a single connection. One such example is a common ground connection for all nozzle channel heaters. Because of the great differential in power demand between the nozzle heaters and the substrate heaters, however, the substrate heaters preferably are electrically charged across a ground terminal separate from the nozzle heaters. In another example, the substrate heater circuit derives voltage source from a bus terminal that also serves the nozzle heaters. Again, however, because of the great power differential and the priority of uniform current flow and timing to the nozzle heaters, it is essential for the substrate heaters not to be energized while the nozzle heaters are operating. Consequently, the substrate heaters may be energized only when the printhead is in transition between print lines. Although an operable prior art compromise, it would be preferable to energize the substrate heater circuit independently of the nozzle heater circuits.
A prior art quality test circuit also may share the same ground terminal with the nozzle heaters. Moreover, test circuits of the prior art may be energized with an independent voltage connection terminal. On the other hand, the quality test circuit has no further function after verification of the finished printhead. Hence, the quality test circuit imposes no current load on the ground terminal common with the nozzle heaters during operation of the printhead. Furthermore, the voltage connection terminal remains unconnected, inactive and unused after final assembly of the printhead with the pen body.
It is an object of the present invention, therefore, to provide printhead substrate heaters with an independent power circuit and connections.
It is also an object of the invention to free the nozzle heater bus circuit from a need to also conduct electrical current to the substrate heaters.
A further object of the invention is to free the operation of printhead substrate heaters from dependence upon the operational status of the printhead nozzle heaters